The invention relates to a magnetic memory cell and a memory comprising at least one magnetic memory cell.
Magnetic random access memories (MRAMs) have been proposed due to their non-volatile nature. Unlike dynamic random access memory (DRAM) cells, non-volatile memory cells such as MRAM cells do not require a complex circuitry for perpetual electronic refreshing of the stored Information.
The first of such MRAMs were based on magnetic multi-layer structures, deposited on a substrate. U.S. Pat. No. 5,343,422, for example, discloses a structure in which two layers of ferromagnetic material are separated by a layer of non-magnetic metallic conducting material. One of the magnetic materials, called the ferromagnetic fixed layer (FMF), has a fixed direction of magnetic moment, e.g., by having a particularly high coercive field or strong unl-directional anisotropy. The other magnetic layer, called the ferromagnetic soft layer (FMS), has a preferred axis for the direction of magnetisation, the so called easy-axis, which is aligned parallel to the magnetic moment of the ferromagnetic fixed layer. The magnetic moment of this ferromagnetic soft layer is free to change direction between parallel and anti-parallel alignment relative to the easy-axis, and as a consequence, also relative to the magnetic moment of the ferromagnetic fixed layer on application of an external magnetic field.
The state of the storage element represents a logical xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d depending on whether the directions of the magnetic moments of the magnetic layers are in parallel or anti-parallel alignment, respectively. Because the resistance of the storage element is different for different mutual orientations of the magnetic moments, the structure acts as a spin valve. It thus allows the sensing of the state of the storage element by measuring the differential resistance xcex94R/R with a current, where xcex94R is the difference in resistance of the storage element for two different states of relative orientation of the magnetic moments, and R is the total resistance of the structure in the lower resistance state.
A switching between these orientations can be achieved by passing write currents in the vicinity of the FMS, usually by using write lines which run past the layered structure on either side. These write currents, which do not pass through the layered structure Itself, induce a magnetic field at the location of the FMS which alters the orientation of the FMS, if it is stronger than the coercive field HC of the FMS.
An alternative is disclosed in U.S. Pat. No. 6,072,718. There, the conducting non-magnetic spacer layer between the two magnetic layers is replaced by an insulator. The device therefore forms a magnetic tunnel junction (MTJ), where spin polarised electrons tunnel through the insulator. The cell disclosed in U.S. Pat. No. 6,072,718 is written by sending simultaneously a current through the word and bit line crossing at the location of the cell. Each of these currents causes a magnetic field at the location of the memory cell. As the word lines and the bit lines are perpendicular to each other, the orientations of the magnetic fields caused by the currents at a crossing point of a bit line and a word line are perpendicular, too. One of both magnetic fields, the so called hard-axis field, extends parallel to the magnetic hard-axis of the ferromagnetic soft layer, while the other one of the magnetic fields, the so called easy-axis field, extends parallel to the magnetic easy-axis of the ferromagnetic soft layer.
In a write process, usually the hard-axis field, which stays perpendicular on the magnetic moment of the ferromagnetic soft layer, is applied to the ferromagnetic soft layer in order to move the magnetic moment out of its actual orientation and the easy-axis field Is used to set the new orientation of the magnetic moment with respect to the easy-axis of the ferromagnetic soft layer.
During a write process, all memory cells arranged in a first line will experience the same hard-axis while all memory cells arranged in a second line perpendicular to the first line will experience the same easy-axis field. The strength of both magnetic fields is chosen such that one of both fields alone is not able to switch a memory cell. Therefore, in an ideal memory array (i.e. all memory cells of the array show the same magnetic response to an applied magnetic field), only the memory cell which is located at the crossing of both lines experiences the hard-axis field as well as the easy-axis field and is therefore written. In contrary to the ferromagnetic soft layer, the ferromagnetic fixed layer has a coercivity that is high enough such that its magnetic moment is left unchanged in this process.
However, in an actual memory cell array, due to many factors related to manufacturing uncertainties and intrinsic magnetic variability, variations in the magnetic response throughout the memory cells in an memory cell array can be very large. Due to these variations, some of the memory cells may already be written if only one of the magnetic hard-axis field and the magnetic easy-axis field is applied. As a consequence, an array wide selectivity of the writing process is generally not achieved. The response variations are e.g. caused by tolerances during the manufacturing process, which for example may lead to differences in the surface roughness of different cells, which has an influence on the coercivity of the cell.
In GB 2 343 308, a magnetic storage device is disclosed, which comprises a first and a second ferromagnetic layer and a tunnel barrier which is disposed between both ferromagnetic layers. The first ferromagnetic layer is a ferromagnetic fixed layer whereas the second ferromagnetic layer is a ferromagnetic soft layer which can change the orientation of its magnetic moment. The device can be written directly by applying a voltage across the cell which can switch the orientation of the magnetic moment of the ferromagnetic soft layer with respect to the ferromagnetic fixed layer. The switching is effected by means of an induced exchange interaction between the ferromagnetic fixed layer and the ferromagnetic soft layer related to spin-polarised electrons tunnelling through the tunnelling barrier. Since the addressing of the cells in the write process is direct, array wide selectivity is achieved.
In GB 2 343 308, it is important for the write process to supply a strong enough tunnelling current to overcome the coercive field of the ferromagnetic soft layer. Therefore, the tunnel barrier has to be as thin as possible. Because the tunnelling current increases exponentially with decreasing thickness of the tunnelling layer, local variations due to the manufacturing process become particularly pronounced for thin barriers. The less uniform the current distribution within the cell, the higher the total current has to be to create a strong enough excitation throughout the entire ferromagnetc soft layer. However, a too strong a current will eventually break the tunnel junction. Therefore, in GB 2 343 308 materials for the tunnelling layer have been proposed with a low energy barrier. Nevertheless, from a manufacturing point of view, there is still a very strong a focus on the quality of the manufacturing process.
It is therefore an object of the invention to provide a memory cell that avoids the drawbacks of known memory cells with respect to writing and reading the cell.
The object is achieved by a magnetic memory cell comprising
a first ferromagnetic fixed (hereinafter FMF) layer with a first magnetic moment,
a second FMF layer with a second magnetic moment,
at least one ferromagnetic soft (hereinafter FMS) layer with a third magnetic moment, said FMS layer being arranged between the first and second FMF layers,
a first non-magnetic intermediate layer arranged between said first FMF layer and said FMS layer,
and a second non-magnetic intermediate layer arranged between said second FMF layer and said FMS layer,
wherein said first intermediate layer is adapted to allow a spin-polarized write current to pass between said first FMF layer and said FMS layer, said write current having an amount sufficient to change a relative orientation of said first and third magnetic moments, and
wherein said second intermediate layer is adapted to influence the resistance between said second FMF layer and said FMS layers at a predetermined read voltage in dependence on a relative orientation of said second and third magnetic moments, said read voltage creating a spin-polarized current amount lower than said write current amount,
and wherein said first and second magnetic moments are in a predetermined parallel or antiparallel alignment relative to each other.
The memory cell of the invention combines the benefits of known types of magnetic memory cells, while eliminating their drawbacks.
The memory cell according to the invention comprises two cell sections: a first cell section has the first FMF layer, the FMS layer, and the first intermediate layer between the first FMF layer and the FMS layer. A second cell section includes the second FMF layer, the FMS layer, and the second intermediate layer between the second FMF layer and the FMS layer. A ferromagnetic fixed layer is a layer with a coercive field high compared to that of a ferromagnetic soft layer.
The cell sections provide different main functionalities in the memory cell of the invention. The first cell section primarily serves for writing the cell. Writing means changing the relative orientation of the magnetic moments of the first and third magnetic moments, that is, of the first FMF layer and the FMS layer. It will be explained below how writing of the memory cell can be accomplished. The second cell section primarily serves for reading the cell. Reading, in the current context, means ascertaining the relative orientation of the second and third magnetic moments.
Since the relative orientation of the magnetic moments of the first and second FMF layers is fixed in the memory cell of the invention, knowing the relative orientation of the second and third magnetic moments implies the knowledge of the relative orientation of the first and third magnetic moments. Therefore, the state of the memory cell as written with the aid of the first cell section can be determined from a measurement of the relative mutual orientation of the second and third magnetic moments in the second cell section.
It is noted that for purposes of clear language the above definition of the memory cell of the invention mentions only once that at least one FMS layer may be provided in the cell. Each further reference to xe2x80x9cthexe2x80x9d FMS layer in the above definition, the description, and in the claims is to one or more FMS layers as well, unless the number of FMS layers is made clear otherwise in the respective context. Thus, according to the invention one or more FMS layers may be provided in the memory cell.
In an embodiment comprising only one FMS layer, this FMS layer is shared by the first and second cell sections. Sharing an FMS layer in this context means that the respective layer is used in the writing as well as in the reading process.
In an embodiment comprising two FMS layers, each cell section preferably has one FMS layer. It is important that, in an embodiment comprising more than one FMS layer, the FMS layers are mutually coupled by a magnetostatic interaction. This coupling effects an antiparallel alignment of the orientation of the FMS layers of the first cell section and the second cell section. This coupling is accomplished by providing a FMS layer in the first cell section that has a magnetic moment sufficiently high to Impose on the magnetic moment of the FMS layer in the second cell section, given a distance between the two FMS layers. A lower magnetic moment in the FMS layer in the first cell section is needed for a smaller distance between the FMS layers. Thus, both parameters can be adjusted to achieve an optimized cell design. However, as a boundary condition, the magnetic moment of the FMS layer in the first cell section has to be small enough to allow changing this magnetic moment with the aid of the writing current in the first cell section.
In the embodiment comprising two FMS layers, the FMS layer in the first cell section is used for writing the memory cell. Writing the FMS layer in the first cell section, however, implies writing the second FMS layer in the second cell section due to the magnetostatc coupling of these two layers. Therefore, the magnetic moment of the FMS layer in the second cell section always reflects the information contained in the magnetic moment of the FMS layer of the first cell section of the memory cell of the invention.
An MTJ cell section is used to read the state of the MTJ cell section, i.e., to ascertain the relative orientation of the magnetic moments of the ferromagnetic fixed and soft layers of magnetic memory cell. The coupling between the cell sections makes sure that both cell sections share the same state. Sharing the same state means that there is a unequivocal relation between the relative orientation of the magnetic moments.
In the following, a number of embodiments based on the above described memory cell of the invention are disclosed.
A first embodiment, wherein respective extensions of said first FMF layer, said first intermediate layer, and said FMS layer in a direction perpendicular to the layer planes, as well as the respective materials of these layers are such as to allow a change of an orientation of said first and third magnetic moments relative to each other with the aid of a current of at least said writing current amount.
A second embodiment, wherein respective extensions of said second FMF layer, said second intermediate layer, and said FMS layer in a direction perpendicular to the layer planes, as well as the respective materials of these layers are such as to form, given a predetermined writing voltage applied across these layers, a low ohmic resistance if the second and third magnetic moments are in parallel alignment, and to form a high ohmic resistance if the second and third magnetic moments are in antiparallel alignment.
A third embodiment, wherein said second FMF layer, said second intermediate layer, and said FMS layer form a magnetic tunnel junction.
A fourth embodiment, comprising a current selection element adapted to allow a current of up to at least a predetermined writing current amount to pass across the first FMF, the first intermediate layer and the FMS layer in directions perpendicular to the layer planes, and a reading current amount to pass across the first second FMF layer, the second intermediate layer and the FMS layer in one direction perpendicular to the layer planes at a predetermined reading voltage.
A fifth embodiment, comprising a first FMS layer and a second FMS layer, wherein the magnetic moments of said first and second FMS layers are coupled to each other by magnetostatic interaction.
A sixth embodiment, wherein said FMS layers are separated by a conductive layer.
A seventh embodiment, comprising a first current selection element and a second current selection element,
said first current selection element contacting said first FMF layer and said second current selection element contacting said second FMF layer,
wherein said first current selection element is adapted to allow a current of up to at least a predetermined writing current amount to pass across the first FMF, the first intermediate layer and the first FMS layer in directions perpendicular to the layer planes, and
wherein said second current selection element is adapted to allow a reading current amount to pass across the second FMF layer, the second intermediate layer and the second FMS layer in one direction perpendicular to the layer planes at a predetermined read voltage applied across these layers.
An embodiment of a memory according to the invention comprises at least one memory cell according to the fifth embodiment, wherein the conductive layer is a word line or a bit line.